Rotary unions that incorporate a rotating fluid seal between the axial mateable sealing surfaces of a pair of relatively rotatable parts thereof are well known in the art. Typical of these rotary unions is a rotary union of the kind for effecting the transfer of fluid from a stationary fluid source to a fluid conduit in the form of a rotating spindle, shaft, clutch hub or other such device into which fluid is to be fed.
A typical fluid rotary union includes a rotor seal member and a stator seal member which are assembled in co-axial relationship in a common housing for relative rotation and passing fluid. The stator and rotor are axially biased towards one another such that the axial sealing surfaces thereof are in engagement and define a rotating seal interface in the housing that is generally perpendicular to the axis of rotation. The rotor seal member is journalled on a bearing for rotation relative to the housing and normally includes a threaded shaft on the line which extends from the housing to be affixed to the rotating spindle for rotation therewith.
These prior art rotary unions must be capable of containing very high pressures while rotating at very high speeds. This is made possible by an almost perfect mating of the sealing surfaces which are disposed generally perpendicular to the axis of rotation. These micro-lapped sealing surfaces must rotate smoothly and easily with a minimum amount of friction to assure long life and still not leak. In order to maintain the sealing at high pressures, these prior art rotary unions utilize the pressure seal principle. A nominal load is applied to the two sealing surfaces by a biasing spring in order to seal the union during times when the fluid within the coupling is not pressurized. As the pressure of the fluid within the coupling increases, the load between the two sealing surfaces increases due to the fluid pressure acting against either the rotor or the stator to force these two components together. The total amount of pressure acting on the rotating sealing surfaces can be controlled by controlling the surface areas exposed to the fluid pressure. This, in turn, will control the amount of friction between the sealing surfaces, the amount of torque to rotate the coupling and thus the wear of the sealing surfaces. While the total amount of load is controllable, these prior art unions maintain a load between the sealing surfaces when the union is not subjected to fluid pressure. The presence of this load means the friction between the sealing surfaces continues to cause a higher torque to rotate the union and additional wear between the sealing surfaces.
Rotary unions have been designed to control this pressure seal principle by incorporating a balanced sealing pressure on the loaded rotor or stator. The balanced sealing pressure on the rotor or stator means that the sealing load on the sealing surfaces is maintained only by the biasing spring with loading by the pressurized fluid being generally eliminated. While this balanced sealing approach has increased the durability of the rotary union, the sealing of the rotary union is somewhat compromised due to the identical sealing load being applied at both low pressure and high pressure conditions of the rotary union. In addition, when these balanced sealing pressure rotary unions are operated without fluid pressure, the load between the sealing surfaces is still present. The presence of this load means the friction between the sealing surfaces continues to cause a higher torque to rotate the union and additional wear between the sealing surfaces.
Accordingly, what is needed is a rotary union which is capable of providing sufficient sealing loads on the sealing surfaces during periods when the rotary union is subjected to fluid pressures to insure a positive seal between the sealing surfaces and is also capable of eliminating the load between the sealing surfaces during periods when the rotary union is not subjected to fluid pressures.